GENERAL ZOOLOGY 



Fig. 6.14. Results of monohybridization of flies which differ in type of wing. The chromosomes 

 are shown as the carriers of the genes; I ' is the symbol used for the gene for long wing, which 

 is dominant to the gene, v, for vestigial wing (cf. Fig. 6.11). (Modified from T. H. Morgan 

 et al.. The Mechanism of Mendelian Heredity, copyright 1922 by Henry Holt and Co., printed 

 by permission.) 



mouse, which will always be homozygous since white is recessive, is presented 

 in Figure 6.12. This is known as a back-cross with a recessive. The difference 

 between the offspring obtained in this cross and those obtained by crossing 

 a homozygous gray with a white mouse (Fig. 6.11) furnishes the type of breed- 

 ing test used for differentiating homozygous from heterozygous dominant 

 individuals. The diagram in Figure 6.13 shows why indiscriminate crossing 

 between the gray mice would fail to yield information that would enable one 

 to distinguish homozygous gray mice with certainty. In the case illustrated 



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